1. Field of the Invention
The present invention relates to impedance measurement and specifically to single port impedance measurement over a broad range of frequencies including radio frequencies.
2. Prior Art
The complex impedance associated with an electrical circuit or component is commonly measured with electronic test equipment. The complex impedance at any specific frequency consists of a real resistive portion and an imaginary reactive portion, expressed in units of Ohms. The combination of resistance and reactance offers opposition to the flow of direct and alternating current. The ratio of the complex voltage to complex current is equal to the complex impedance.
Most prior art approaches to impedance measurement use directional-couplers. FIG. 1 shows a prior art complex impedance measurement method using a directional-coupler. The forward and reverse coupled signals reflecting from the load are measured to compute the impedance of the unknown test load.
Moehlmann, U.S. Pat. No. 5,206,600 entitled “Impedance determining apparatus using quadrature current and peak detectors”, incorporated herein by reference, discloses the use of directional-couplers to sample the voltage and current of a signal passing through a transmission line and derives the complex impedance parameters independently of the amplitude or phase of the signal.
De Santix, U.S. Pat. No. 4,071,819 entitled “Device for sensing the real part in a complex impedance”, incorporated herein by reference, uses directional-couplers to measure the real part of a load impedance.
Another approach found in the prior art for impedance measurement is to measure an input excitation signal and an output signal from the device under test. One limitation with this approach is that it is only usable with a two port-device, having an input port and an output port. Tamamura, U.S. Pat. No. 4,860,227 entitled “Circuit for measuring characteristics of a device under test”, incorporated herein by reference, discloses one embodiment of this approach, where an excitation signal is applied to the input port, with the output signal and the excitation signal frequency-converted down in frequency and sampled by A/D converters.
Hall U.S. Pat. No. 4,242,631 entitled “Front-end circuit apparatus for impedance measurement and the like”, incorporated herein by reference, discloses the use of a floating transformer connected across series-coupled known and unknown impedances with grounding switches that alternately connect one terminal of the impedances to ground. The common node between the impedances is sampled to measure the unknown impedance.
Another approach to impedance measurement is to use pulse excitation of the unknown load. One example is shown in Bottman, U.S. Pat. No. 5,633,801 entitled “Pulse-based impedance measurement instrument”, incorporated herein by reference, which discloses a handheld device that generates a stimulus pulse to a device under test, a measurement of the pulse response, and FFT analysis of the measured signal to determine the complex impedance of the device under test. A limitation of this approach is that the device is not tested with a signal that is representative of the type of signals normally used in the circuit's operation. Additionally, the pulse-based approach has practical limits to the frequency range that can be tested. It is preferable to test the device in question at the frequency or range of frequencies of interest rather than the frequency content represented by the pulse excitation.
A limitation of the prior art approaches to impedance measurement is that the parameters of the directional-coupler used for measurement vary with frequency, thus introducing errors into the impedance measurement of the unknown impedance. The directional-couplers are also designed for optimum performance at a specified load resistance. Connecting to impedances that are different from the specified load resistance produces secondary effects that can impair the accuracy of the directional-coupler signal measurements. The design difficulty and cost of the directional-coupler can also be significant particularly for broadband applications covering a multi-octave frequency range.